1. Field of the Invention
The present invention relates generally to fingerprint scanning and imaging.
2. Related Art
Biometrics are a group of technologies that provide a high level of security. Fingerprint capture and recognition is an important biometric technology. Law enforcement, banking, voting, and other industries increasingly rely upon fingerprints as a biometric to recognize or verify identity. See, Biometrics Explained, v. 2.0, G. Roethenbaugh, International Computer Society Assn. Carlisle, Pa. 1998, pages 1-34 (incorporated herein by reference in its entirety).
Fingerprint scanners having image sensors are available which capture an image of a fingerprint. A signal representative of the captured image is then sent over a data communication interface to a host computer for further processing. For example, the host can perform one-to-one or one-to-many fingerprint matching.
Due to manufacturing tolerances and techniques, image sensors can vary from chip-to-chip, column-to-column, row-to-row, or even from pixel-to-pixel within a given chip. For example, a particular column within an image sensor may have a greater, or lesser, sensitivity to light (xe2x80x9csensitivity offsetxe2x80x9d) than the average sensitivity among the remaining columns. Since this sensitivity offset is particular to the image sensor itself, a column having an offset in a first image sensor may not have an offset in a second image sensor. As a result of sensitivity offsets, an image sensor excited with a uniform light source may exhibit non-uniform illumination among columns. This results in a non-uniform image. Furthermore, as a result of variances among image sensors, two image sensors excited by the same uniform light source may produce non-uniform images which differ from one another. This image distortion particular to the image sensor itself is referred to as fixed pattern noise (xe2x80x9cFPNxe2x80x9d).
While low levels of FPN are sometimes acceptable in an image sensor within a fingerprint image scanner, this noise may be of a magnitude sufficient to degrade image quality to an unacceptable level. Furthermore, acceptable FPN levels may become unacceptable if, for example, the gain of the image sensor is increased to accommodate lower light levels.
It is therefore desirable to produce a fingerprint scanner having accurate image output despite the FPN associated with the particular image sensor used in the fingerprint scanner. Furthermore, since mobile use is increasingly desired in biometric applications, such as law enforcement, police and others need a portable hand-held device to easily and accurately capture fingerprint images in the field. It is therefore desirable to produce a mobile, hand-held fingerprint scanner having accurate output despite the FPN associated with the particular image sensor used in the fingerprint scanner.
The present invention provides a fingerprint scanner which includes image normalization data stored locally with the image sensor. Likewise, the present invention provides a fingerprint scanning system which includes remote fingerprint scanners having local storage of image normalization data. Such local storage of image normalization data is advantageous because it allows for interchangeability among remote scanners as well as between remote scanners and host processors.
In a first embodiment, a remote fingerprint scanner, which may be a mobile, hand-held unit, includes an image sensor and image normalization data. The image normalization data is stored in a non-volatile memory. The non-volatile memory is disposed locally with the image sensor. The image normalization data is specific to the image sensor and is representative of non-uniform sensitivity among pixels, columns of pixels, or rows of pixels within the image sensor. Additionally, the remote fingerprint scanner may include a data communications interface, such as a universal serial bus (USB) or an IEEE 1394 (also called FIREWIRE) compatible interface, that couples data between the remote fingerprint scanner and a docking station. Furthermore, the remote fingerprint scanner also may include a camera board on which are disposed the image sensor as well as the non-volatile memory. In the first embodiment, the output of the remote fingerprint scanner includes both raw image data as well as the image normalization data. This allows for image normalization, and thus the elimination of fixed pattern noise, at the host processor rather than within the remote fingerprint scanner. Thus, in the first embodiment, the remote fingerprint scanner does not include a normalization processor.
In a second embodiment, a remote fingerprint scanner also includes a normalization processor. The normalization processor is preferably included in the remote fingerprint scanner""s system controller. The normalization processor normalizes image output. Thus, a remote fingerprint scanner in the second embodiment outputs a normalized image. Alternatively, even with the normalization processor, the remote fingerprint sensor of the second embodiment may be configured so that its output includes raw image data together with image normalization data, like the output of the remote fingerprint scanner of the first embodiment.
In addition to the fingerprint scanners of the first and second embodiments, a fingerprint scanning system is provided. Such a system includes a host processor in addition to at least one fingerprint scanning unit corresponding to the first or second embodiments and may also include a docking station as well a data communications interface.
Also disclosed is a method of manufacturing a remote fingerprint scanner according to the first and second embodiments. The disclosed method includes the steps of exciting an image sensor with a light source determining a set of normalization coefficients. These normalization coefficients are then stored in a non-volatile memory. The non-volatile memory may be disposed locally with the image sensor. The light source used in the method may be a uniform light source. The intensity of the uniform light source may be adjusted so as to avoid saturation at either extreme of sensitivity of said image sensor.
The step of determining a set of normalization coefficients may further include determining an average image intensity of a predetermined area of pixels within the image sensor the image sensor. The predetermined area of pixels may be a plurality of columns within the image sensor.
Likewise, the step of determining a set of normalization coefficients may also include determining an average column intensity for each column within said image sensor. Furthermore, comparing an average column intensity for each column to an average image intensity may also be included in the step of determining a set of normalization coefficients. Such a comparison resulting in a normalization coefficient for each column. Preferable equations used in the manufacturing method are also disclosed.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.